fix: cli: return an error when creating a secret that already exists (#2408)

[g-linville]

for #2408

Pretty simple little fix.

Checklist

• [x] The title of this PR would make a good line in Acorn's Release Note's Changelog
• [x] The title of this PR ends with a link to the main issue being address in parentheses, like: This is a title (#1216). Here's an example
• [x] All relevant issues are referenced in the PR description. NOTE: don't use GitHub keywords that auto-close issues
• [x] Commits follow contributing guidance
• [ ] Automated tests added to cover the changes. If tests couldn't be added, an explanation is provided in the Verification and Testing section
• [x] Changes to user-facing functionality, API, CLI, and upgrade impacts are clearly called out in PR description
• [ ] PR has at least two approvals before merging (or a reasonable exception, like it's just a docs change)

[g-linville]

The integration test that is failing here also fails on main, so I'm just going to merge this.